L] , APPENDIX. 63 



death of the cells themselves. But the fact that such functions 

 have appeared, — involving as they do the sacrifice of a great 

 number of cells, — entirely depends upon the replacement of 

 the old by newly formed cells, that is by the process of repro- 

 duction in cells ^ 



We cannot a priori dispute the possibilit}^ of the existence of 

 tissues in which the cells are not worn out by the performance 

 of function, but such an occurrence appears to be improbable 

 when we recollect that the cells of all tissues owe their con- 

 stitution to a very far-reaching process of division of labour, 

 which leaves them comparatively one-sided, and involves the 

 loss of many properties of the unicellular, self-sufficient or- 

 ganism. At any rate we only know of potential immortality in 

 the cells which constitute independent unicellular organisms, 

 and the nature of these is such that they are continually under- 

 going a complete process of re-formation. 



If we did not find any replacement of cells in the higher 

 organism, we should be induced to look upon death itself as the 

 direct result of the division of labour among the cells, and to con- 

 clude that the specific cells of tissues have lost, as a consequence 

 of the one-sided development of their activities, the power of 

 unending life, which belongs to all independent primitive cells. 

 We should argue that they could only perform their functions 

 for a certain time, and would then die, and with them the 

 organism whose life is dependent upon their activity. The 

 longer they are occupied wdth the performance of special 

 functions, the less completely do they carry out the phenomena 

 of life, and hence they lead to the appearance of retrogressive 

 changes. But the replacement of cells is certain in many 

 tissues (in glands, blood, etc.), so that we can never seek a 

 satisfactory explanation in the train of reasoning indicated 

 above, but we must assume the existence of limits to the re- 

 placement of cells. In my opinion, we can find an explanation 

 of this in the general relations of the single individual to its 

 species, and to the whole of the external conditions of life ; and 



' Roux, in his work •' Der Kampf der Theile im Organismus,' Jena 

 1881, has attempted to explain the manner in which division of labour 

 has arisen among the cells of the higher organisms, and to render in- 

 telligible the mechanical processes by which the purposeful adaptations 

 of the organism have arisen. 



